Semiconductor devices are manufactured by the microfabrication technology based on photolithography. In the photolithography, a photoresist layer is formed on a silicon wafer. Using an exposure apparatus, an image on an original plate known as a reticle or mask is transferred to the photoresist layer, which is developed into a resist pattern. Then the silicon or a metal or another material underneath the resist pattern is etched for forming an electronic circuit on the silicon wafer. In order to form a pattern of finer size for further integration of semiconductor devices, efforts have been made to reduce the wavelength of the exposure light used in the photolithography. In the mass production process of 64 Mbit DRAM, for example, KrF excimer laser (248 nm) is utilized. For the fabrication of DRAMs requiring a finer patterning size of 0.13 μm or less, ArF excimer laser (193 nm) is utilized. It is under investigation to fabricate 65-nm node devices by combining light of such shorter wavelength with a lens having an increased NA of 0.9. For the fabrication of next generation 45-nm node devices, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are adopted due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography is abandoned, and the ArF immersion lithography is now under study.
In the photolithography wherein a photoresist layer is exposed through a reticle, the moving stage on which a wafer rests is finely moved in the exposure apparatus in a projection light axis direction, so that the wafer surface may be in register with the best image plane of the projection optical system, that is, so as to enhance focus. Used as a sensor for such focusing is an optical focus detection system of the off-axis illumination type in which an imaging light flux (of non-exposure wavelength) is obliquely projected onto the wafer surface and the reflected light is detected, as disclosed in JP-A S58-113706. The imaging light flux used for this purpose is infrared light, especially near-infrared light, as disclosed in JP-A H02-54103, JP-A H06-29186, JP-A H07-146551, and US 20090208865.
The exposure apparatus using infrared light in the focus detection system suffers from the problem that an exact focus cannot be detected because infrared light is transmitted by a photoresist layer. That is, part of infrared light for focus detection is transmitted by the photoresist layer, the transmitted light is reflected by the substrate surface and enters the detection system along with the light reflected by the wafer top surface. As a result, the accuracy of focus detection is degraded.
The optical auto-focusing is such that the position of the top surface of the wafer is determined by reflecting infrared light on the wafer top surface and detecting the reflected light, after which the wafer is driven so as to fall in register with the imaging plane of the projection lens. Apart from the light reflected by the wafer top surface, there is present light that is transmitted by the resist layer and reflected by the substrate surface. If detection light having a certain band of light intensity distribution enters the detection system, the position measurement value represents the center of the light intensity distribution, leading to the degraded accuracy of focus detection. In general, the substrate has a multilayer structure including patterned metal, dielectric material, insulating material, ceramic material and the like, and the patterned substrate makes reflection of infrared light complex so that focus detection may be difficult. If the accuracy of focus detection is degraded, the projected image becomes vague to detract from the contrast, failing to form a satisfactory photoresist pattern.
To increase the accuracy of optical auto-focusing near infrared light, JP-A H07-146551 proposes the use of a photoresist layer containing a near-infrared absorbing dye. In this case, near-infrared light is not transmitted by the photoresist layer, and no reflected light other than the light reflected by the wafer top surface enters the focus detecting system, and as a result, the accuracy of focus detection is improved. However, since the near-infrared absorbing dye used therein should not be one that absorbs exposure light or degrades the resolution of a photoresist film, it is least amenable to the photolithography using ArF excimer laser. US 20090208865 proposes a method for introducing a near-infrared absorbing dye-containing layer below a photoresist layer, which method can prevent degradation of the resolution of the resist.
One alternative to the optical autofocus technique is a method based on the principle that detects the pressure of air discharged onto the wafer surface, known as Air Gauge Improved Leveling (AGILE™). See Proc. of SPIE Vol. 5754, p. 681 (2005). Albeit excellent accuracy of position measurement, this method takes a long time for measurement and is not accepted in the mass production of semiconductor devices requiring improved throughputs.
It would be desirable to have a method capable of brief accurate auto-focusing in optical lithography.